The widespread use of cellular telephones and other compact or portable RF communication devices such as toll-tag readers, identification card readers, and devices for scanning items in inventory has resulted in intense interest in employing antennas with high efficiency and compact size. The early implementations of mobile cellular telephony devices were of lunchbox size or larger, and required a power level that generally required a substantial power source such as provided by an automotive alternator and battery. However, as cellular technology has evolved with paralleling reductions in size and power requirements, cellular telephones and other portable communication devices have become small enough to fit easily into the palm of one's hand, and can be operated for practical periods of time from a small internal rechargeable battery. Similarly, scanners for recognizing tagged items in inventory have become very compact and portable.
Over the years of development of radio and related telecommunication technologies, numerous antenna configurations have been developed. An antenna is a circuit element configured to convert RF (radio frequency) energy flowing in circuit conductors into a radiated form that can propagate freely in space. An antenna exhibits reciprocal properties in that the same physical configuration can receive as well as transmit radiation with substantially similar characteristics.
A basic antenna configuration is a dipole which is a conductive line, insulated at both ends, coupled to an RF power source near, but not necessarily at, its center. A monopole antenna is a variation of a dipole antenna that consists of half a dipole adjacent to a conductive plane configured to provide a mirrored electromagnetic field that functionally replaces the missing dipole half. An alternative to a dipole is a conductive loop of wire, also fed from an RF power source coupled to the wire ends.
Further variations of these antenna configurations include the addition of directive and reflective conductive elements that provide directivity to the radiated signal from the antenna, parabolic conductive surfaces to focus the radiated beam, waveguide termination configurations, microstrip lines, and combinations of these approaches.
From a design perspective, an antenna is required to exhibit a number of characteristics to make it a practical circuit element for use with a communication device. One characteristic is that it exhibits reasonable “gain”, which relates to its radiation directivity and efficiency. Directivity refers to the directional variation of its transmitting and receiving properties. Relatively omnidirectional transmitting and receiving characteristics are often desired for portable communication devices, which avoid the need for the user to maintain an orientation of the device in a particular direction while communicating. Small dipole and loop antennas inherently exhibit substantially omnidirectional transmitting and receiving characteristics.
Efficiency refers to the fraction of power that is radiated compared to the total power delivered to the antenna, a portion of which is lost in the resistance of conductive elements and dielectric media. The need for high efficiency is related to the use of smaller batteries and smaller power processing circuit elements, since the amount of RF power that would otherwise have to be generated can be reduced. Efficiency is important because batteries make a significant cost and size contribution to the design of cellular telephones.
Another property of interest is the antenna input impedance. This refers to the ratio of voltage to current, including any phase difference that is applied to the terminals of the antenna, and affects possible need for additional circuit components that would otherwise be included for efficient coupling of power to the antenna. Antenna bandwidth refers to the variation of any property over a range of frequencies, and is an indication of the antenna's utility for a particular band of frequencies that may be allocated for its intended use. Antenna bandwidth is an important characteristic of antennas intended for use in portable communication devices because the assigned frequency bands may often have a bandwidth that is 6%–8% or more of the nominal transmission frequency. Antenna bandwidth is particularly important for antennas that are small in relation to their wavelength because of the generally low efficiency of such relatively small antennas.
As the size of cellular telephones has been reduced, the size of the antenna has also been reduced. Early cellular telephones utilized a monopole antenna about a quarter wavelength in length, which was often retractable within the body of the communication device when not in use. Since the present frequency bands for cellular communication are at about 1 and 2 GHz, the corresponding length of an extended monopole antenna is about 3.2 or 1.6 inches, respectively. This has been a practical arrangement for some early portable telephones, but the continuing pressures of the marketplace provide advantage to products with antennas of even smaller size.
Microstrip antennas, which consist of a conductive strip on an insulating substrate applied over a conductive surface, have been an important step in reducing antenna size because of the absence of a mechanical structure projecting from the end of the telephone, such as a monopole antenna. A microstrip antenna can effectively be a layered structure on a surface of the telephone requiring little volume without compromising good transmitting and receiving performance. Nonetheless, the length of the conductive layer has been required to be on the order of a quarter wavelength in order to achieve reasonable antenna performance as measured by input impedance, antenna gain, bandwidth, or other parameter required by the design. Microstrip length has become a limitation as cellular telephones continue to shrink. In general, most antennas exhibit a compromise in performance when their size is substantially smaller than a quarter wavelength of the transmitted or received signal.
Telephones incorporating monopole and microstrip antennas are described in U.S. Pat. No. 6,633,262 (Shoji, et al.), U.S. Pat. No. 6,628,241 (Fukushima, et al.), U.S. Pat. No. 6,281,847 (Lee), and U.S. Pat. No. 6,133,878 (Lee), which are incorporated herein by reference.
With widespread utilization of cellular telephones, a new characteristic, specific absorption rate (SAR) has become a parameter of great importance. SAR refers to the power absorbed in adjacent tissues of the head during transmitting operation of a cellular telephone. SAR represents a perceived risk for long-term exposure of head tissues as a consequence of the deep penetration of RF radiation in tissues of biological origin at frequencies used for cellular communication. Thus, it is desirable that SAR be reduced as much as possible. SAR is already a characteristic that is limited for cellular devices sold in certain countries such as Japan and Korea, and SAR may also become limited in devices sold in the U.S. As general uses for compact and portable transmitters become widespread, personally absorbed radiation will become an issue of greater interest and concern.
Design directions that can be taken to limit SAR are reduction in transmitted power, which is undesirable because it limits the useful range of the telephone or other transmitting device, locating the antenna farther from a person's head or other body part so as to reduce personal exposure to RF energy, which raises marketability issues for cellular telephone and other portable or compact products, increasing antenna efficiency so that less power is required to operate the telephone or other communication device, which is presently a design challenge for small antennas, and possibly altering the configuration of the antenna and its adjacent structures to reduce strength of the near-field radiation adjacent the user's head or other body part without adversely affecting the antenna radiation pattern or other antenna attributes such as antenna gain, size, or input impedance.
There has been extensive research to make microstrip antennas more suitable for use particularly as cellular telephone antennas, mainly because the conducting ground plane may partially shield electromagnetic radiation of the near-field area on the backside of the ground plane, where a user's head is likely to be located. As the size of the ground plane is reduced, its effectiveness at reducing the near field on the second side of the ground plane is correspondingly reduced. A popular technique for size reduction of microstrip antennas is to use thin vertical conductors connecting the radiating patch and the ground plane as in a PIFA (planar inverted F-antenna). However, as indicated above, antenna size has not been reduced beyond a certain level without compromising antenna performance. In many practical applications, as in cellular telephones, such limited size reduction may not be sufficient.
Accordingly, there are needs in the art for new methods and apparatus for configuring an antenna that is usable with portable or compact communication equipment, that can be configured in sizes significantly less than a quarter wavelength, yet preserve electrical characteristics of longer antennas such as input impedance, gain, and efficiency. Such antennas must be operable over frequency ranges that may extend to 6%–8% or more in bandwidth. In addition, the new antenna configuration should exhibit reduced SAR for absorption of electromagnetic energy in adjacent tissues of the head or in proximate surrounding surfaces that are likely to be exposed during intended operation of the device.